Electro-optical oscillators (EOO) have many applications in such areas as optical and wireless communications, radar, RF over fiber, sensing, and imaging. Low phase noise compact mm-wave and sub-mm-wave signal generation is used in portable or non-portable spectroscopy systems, high resolution imaging systems, and small form-factor radars.
FIG. 1 is a simplified block diagram of an electro-optic oscillator 10, as known in the prior art. The optical signal generated by laser 12 is modulated using Mach-Zhender modulator (MZM) 30 to generate modulated signal A_mod. MZM 30 includes a combiner 14 and an optical phase modulator 16. Modulated signal A_mod is delayed using a low-loss optical delay component 16 and converted to an electrical signal I via photo-diode 18. Current signal I is filtered using filter 20, and amplified using amplifier 22 before being fed back to the optical phase modulator 16 disposed in MZM 30.
The noise contribution by different sources in electro-optic oscillator 10 may be modeled as:i2n,total=i2n,electrical+i2n,laser,RIN+i2n,Photodiode,shotwhere in,electrical, in,laser,RIN, and in,Photodiode,shot respectively represent the total input referred current noise of the electronic components, the equivalent current noise corresponding to the laser relative intensity noise (RIN), and the photodiode shot noise, respectively. The −3 dB linewidth of the power spectral density of the generated electrical oscillatory voltage may be defined as:
  C  ≈                    i                  n          ,          total                2            _              2      ⁢                          ⁢              R        2            ⁢              P        0        2            ⁢              τ        2            ⁢                        J          1          2                (                                            V              0                                      V              π                                ⁢          π                )            where R, P0, τ, V0, and Vπ are the photodiode responsivity, the laser power, the delay of the optical delay line, the oscillation amplitude, and the modulator voltage to phase conversion gain, respectively. The term J(.) represents the Bessel function of the first kind